This invention relates to color separation elements and image display devices. More particularly, this invention relates to color separation elements with high color separation capability and image display devices for displaying colors by using such an element.
Color display devices using only one diffraction grating have been disclosed, for example, in Japanese Patent Publications Tokkai 62-293222 and 6-222361. FIG. 1 shows an example of prior art color image display device 1 comprising a surface light source (back-lighting) device 3 for emitting a beam of light which is approximately parallel to its light-emitting surface 2, a single diffraction grating 4 which serves as a color separation element, a microlens array 5 for collecting light on picture elements on a liquid crystal display panel 6 and a diffusion plate 7. When a parallel beam of white light W is emitted from the light-emitting surface 2 of the light source device 3, it is separated by the diffraction grating 4 into red light R, green light G and blue light B and is diffracted into different directions. Since these beams of light with different colors are collected (focussed) at different positions by passing through the microlens array 5, it is possible to form an image display device for a color display without using any color filter if the liquid crystal display panel 6 is arranged such that its picture elements (or the element openings) are at the focal positions of each color. In FIG. 1, as well as in the following figures, the beam of red light R is shown by dotted lines, that of green light is shown by solid lines and that of blue light is shown by broken lines.
With an image display device thus structured, however, the spectral characteristics are not sufficiently strong and the angles between the optical axes of light beams corresponding to different colors are relatively small because only one diffraction grating is used for color separation. As a result, it often happens, as shown in FIG. 2, that beams of red light B, green light G and blue light B reach the next picture element from the ones where they are intended to be focussed. For example, a portion of the red light R passing through the microlens array 5 is made incident onto the neighboring blue and green picture elements, adversely affecting the picture quality of the formed image.
If the spectral characteristics of the diffraction grating 4 are not sufficiently strong, furthermore, the beams of red, green and blue light focussed by the microlens array 5 are made incident also on black matrix areas 9, and this means a loss of light or that there is less for forming the image.
One of the methods for preventing poor picture quality for such reasons has been to make use of a microlens array 5 with a long focal length, as shown in FIG. 3, such that the focal points for the red light R, the green light G and the blue light B are sufficiently separated. By such a method, however, the distance between the microlens array 5 and the liquid crystal panel 6 becomes large. This has the unfavorable effect of increasing the thickness of the image display device 1 as a whole.
Japanese Patent Publication Tokkai 4-60538 disclosed a color image display device using dichroic mirrors but dichroic mirrors are relatively expensive and do not contribute to the miniaturization of the image display device.